Continuous production of biaxially oriented, crystalline, thermoplastic film



April 25, 1967 w ET AL 3,315,308 CONTINUOUS PRODUCTION OF BIAXIALLYORIENTED,

CRYSTALLINE, THERMOPLASTI C FILM Original Filed Feb. 15 1962 2Sheets-Sheet 1 v uzifixwm E INVENTORS' F.E. WILEY H C WAIN April 25,1967 F. E. WILEY ET AL 3,315,303

CONTINUOUS PRODUCTION OF BIAXIALLY ORIENTED,

CRYSTALLINE, THERMOPLASTIC FILM Original Filed Feb. 15, 1962 2Sheets-Sheet 2 INVENTORS FE. WILEY H.C. WAIN United States Patent Thisapplication is a divisional of Ser. No. 173,557, filed Feb. 15, 1962,now Patent No. 3,248,463.

This invention relates to a process for making biaxially oriented filmfrom crystalline thermoplastic polymer. In another aspect it relates toapparatus which can be used for the continuous production of tough,thermoplastic films having balanced properties in desired proportions.

It is well known that molecular orientation of various crystallizablethermoplastic polymers in the form of sheets, films, filaments, tapes,tubes, pipe, or the like, increases the tensile strength of thesestructures. This orientation is commonly brought about by stretching thepolymeric structure after formation thereof, and this stretching shouldbe carried out at temperatures sufliciently low that the polymer is in asubstantially crystalline condition. In other .words, if the temperatureof the polymer is sufficiently high that substantially all of thecrystallites have melted, very little orientation occurs when thestructure is stretched. Numerous methods have developed for thecontinuous production of oriented polymeric structures by extruding thepolymer in the shape desired, subsequently cooling it to a temperaturebelow the temperature required for the formation of crystals, andthereafter stretching the structure by placing it under tension. Biaxialorientation can be brought about by stretching the sheet or film firstin one direction and then in a second direction at approximately rightangles to the direction of the initial stretch. This sequentialstretching is not desirable for many materials, particularly thecrystalline olefin polymers such as polyethylene, polypropylene, and thelike, since the beneficial results obtained in the initial stretch areconsiderably diminished by the second step of the orientation.Simultaneous biaxial stretching is desirable for the production ofbiaxially oriented films of these polymers.

While most of the effort in this field has been directed to increasingthe tensile strengths of polymeric films in one or both directions, wehave found that the biaxially oriented films presently available fromthese procedures are not necessarily satisfactory for heavy packagingneeds, for example, as bag materials for bulk chemical and foodproducts. Because of the very high protection offered by films of olefinpolymers, these being substantially impervious to moisture and highlyresistant to chemicals, bags formed from these films are well suited forthe storage and shipping of chemicals such as fertilizers, for example,ammonium nitrate or ammonium sulfate. These materials are commonlypackaged in bags of 40, 80, or 100 pounds and are inevitably subjectedto rough handling before the product is used by the consumer. -Sucl1bags are most likely to fail when subjected to shock or heavy impactwhich produces stresses that cannot be rapidly dissipated, therebycausing the bag wall to rupture.

We have now discovered a method for continuously producing a biaxiallyoriented film of crystalline thermoplastic polymer in such a manner thatthe film has a balance of properties making it highly suitable for useas .bag material in the heavy packaging field. According to this methodthe polymer melt is extruded in the shape of a tube which is then cooledand reheated to a tem- 3,315,308 Patented Apr. 25, 1967 perature withina few degrees below the crystalline melting point of the polymer,thereby placing it at the orientation temperature. The tube is thenstretched biaxially by simultaneous radial expansion and linearextension and then cooled to set the orientation. A feature of ourinvention comprises directing a current of cooling gas onto the outersurface of the tube as it is being stretched biaxially. The use of thiscooling gas establishes a minimum temperature gradient over theexpanding bubble and the film thus produced has a balance of tensile andelongation properties in both the machine and the transverse directionswhich makes it tough and highly resistant to rupture on impact. Thisbalance of properties which is highly desirable in heavy bag material isnot obtained under otherwise identical conditions but in the absence ofthe current of cooling gas on the outer surface of the expanding tube.

As another aspect of our invention we have provided apparatus forproducing biaxially oriented film of crystalline thermoplastic polymercomprising, in combination, extrusion means including a tubing die, asizing andcooling sleeve attached to said die, a cooling bath positionedto receive the extruded tube from the sizing sleeve, means for pullingthe tube through the sizing and cooling bath, a heating bath positioneddownstream from the pulling means, this heating bath being equipped withmeans for circulating a heated fluid in direct heat exchange with thetube in order to bring the tube to orientation temperature, means forintroducing a pressurized gas inside the tube in order to inflate it asit issues from the heating bath, means for collapsing and pulling theinflated tube in .order to stretch the tube lengthwise as it issues fromthe heating bath, means for chilling the inflated and stretched tube inorder to set the orientation prior to collapsing the tube, and means fordirecting a current of cooling gas onto the outer surfaces of the tubeas it is being inflated and stretched.

It is an object of our invention to provide a method for continuouslyproducing a biaxially oriented film of thermoplastic crystallinepolymer. It is another object of our invention to provide a method ofproducing a film of themoplastic polymer in such a manner that the filmhas a balance of physical properties making it particularly suitable foruse as a bag material. Still another object of our invention is toprovide a tough thermoplastic film which is highly resistant to ruptureon impact. Another object of our invention is to provide apparatus whichcan be used to produce such a film. Other objects, advantages andfeatures of our invention will be apparent to those skilled in the artfrom the following discussion and drawings in which:

FIGURE 1 is a diagram showing the sequence of operations in the filmforming and orientation process;

FIGURE 2 is a drawing in elevation and partly in section of the tubeextruder die head, sizing and cooling sleeve and cooling bath, includinga diagrammatic representation of the apparatus used to supplypressurized gas for the sizing and expansion operations;

FIGURE 3 is a drawing in elevation and partly in section of the tubepulling means and the reheating bath;

FIGURE 4 is an end view of the tube pulling means;

FIGURE 5 is an elevational drawing partly in section of the apparatusused for draft cooling the expanded .to control the flow of cooling airover the outer surface of expanding tube.

While this invention can be used advantageously in the fabrication ofany crystallizable thermoplastic polymer such as polyvinylidinechloride, nylon, polyethylene glycol terephthalate or the like, it is ofparticular advantage in the biaxial orientation of the highlycrystalline olefin polymers, such as polyethylene, polypropylene,poly-l-butene, poly-4-methylpentene l and other homopolymers andcopolymers of similar 'mono-l-olefins containing up to 8 carbon atomsper molecule. We prefer to practice the invention with the morecrystalline olefin polymers, for example, those having a degree ofcrystallinity of at least 70 and more preferably at least 80 percent at25 C. Examples of such polymers are crystalline polypropylene andpolybutenes and the high density ethylene polymers, particularly thehomopolymers of ethylene and copolymers of ethylene with highermono-l-olefins, these polymers having a density of about 0.940 to 0.990gram per cubic centimeter at 25 C. As used herein the term densityrefers to the weight/unit volume (grams/ cubic centimeter) of thepolymer at 25 C. The density of polymer should be determined while thesample of the polymer is at thermal and phase equilibrium. In order toinsure this equilibrium it is desirable to heat the sample to atemperature 15 to 25 centigrade degrees above its melting point andallow the sample to cool at a rate of about 2 centigrade degrees/minuteto the temperature at which the density is to be measured. Any standardmethod for determining the density of a solid can be used. Thecrystallinity ofrthe olefin polymers can be determined by X-raydiffraction or nuclear magnetic resonance. Prior to the determination ofcrystallinity it is desirable that the sample of the polymer be treatedfor thermal equilibration in a manner described in connection with thedensity determination. a

The higher crystalline olefin polymers referred to above do not have asingle freezing and melting point but instead have a crystallinefreezing point at which maximum crystalline formation occurs uponcooling of the molten polymer and a separate crystalline melting pointat which evidence of crystallinity disappears upon heating a sample ofthe polymer from a cooled crystalline condition. Ordinarily the lattertemperature is several degrees above the crystalline freezing point. Thecrystalline freezing point of these polymers can be determined bymelting a sample of the polymer, inserting a thermocouple in the moltenpolymer and allowing the polymer to cool slowly. The temperature isrecorded and plotted on a chart versus time. The crystalline freezingpoint is the first plateau in the time-versus-temperature curve. Forpolyethylene having a density of about 0.960 the crystalline freezingpoint is about 252 F. The crystalline melting point of these polymerscan be determined by melting a small piece of plastic (usually film)under crossed polaroids in a microscope equipped with means for heatingthe polymer. The specimen is heated slowly and the melting point is thetemperature at which birefringence disappears. For polyethylene having adensity of about 0.960 the crystalline melting point is ordinarily about272 F.

temperatures of about 350 to 400 F. are frequently employed. The tubehaving a predetermined diameter and wall thickness issues from theextruder die and passes immediately into a sizing sleeve where it iscooled by indirect heat exchange with a cooling liquid and at the sametime stretched slightly to produce the desired wall thickness. In sizingoperation 11 at least the surface of the tube is cooled to asubstantially crystalline condition, generally at least several degreesbelow the crystalline freezing point of the polymer. With the highdensity ethylene polymers, at least the surface of the tube is cooled tobelow about 250 F. Sinceit is necessary to insure that all of thepoly-mer in the tube is in substantially uniform crystalline condition,the tube is then passed to a cooling step 12 where it is placed indirect heatexchange with a cooling liquid for a suflicient period oftime to cool all of the polymer in the tube below the crystallinefreezing point. Ordinarily the tube is further cooled in this operationto temperatures of about 210 F. or below. The cooled tube then passes toconveying operation 13 which employs a positive-grip conveying meanswhich pulls the tube through the sizing and cooling steps at a rateslightly faster than the extrusion rate. In this way the wall thicknessof the tube can be controlled within relatively narrow limits.

Conveying operation 13 not only pulls the tube through sizing andcooling steps 11 and 12 but also pushes the tube into reheating step 14.The reheating step brings the tube to the proper orientationtemperature, which, as pointed out previously, is within a few degreesbelow the crystalline melting point of the polymer. As the tube issuesfrom reheating step 14 it is subjected to step 16 which includessimultaneous expansion and drawing in combination with the draftcooling. In operation 16 the tube of polymer is simultaneously stretchedin the machine in transverse directions while at the same time it issubjected to a cooling gradient so that the temperature of the tube whenit reaches its final diameter is several degrees below the temperatureof the tube as it issues from reheating step 14. This cooling gradienthas been found essential in the production of films of crystallineolefin polymers having a predetermined balance of properties, forexample, equal elongation properties in both the machine and transversedirections. After the tube has been expanded to its desired diameter itis immediately chilled in step 17 to reduce the temperature of thepolymer to substantially below its orientation temperature so that nofurther stretching takes place in either the machine or transversedirections. The expanded and chilled tube is then collapsed in step 18to form a two layer film The optimum temperature for orientation is thehighest temperature which can be achieved while the resin mass is stillin a substantially crystalline condition. This temperature will varydepending upon the polymer used and its crystalline melting point. Forease of control it is desirable that this temperature be approached frombelow by heating a film of the polymer which is in a substantiallyuniform crystalline state. Nonuniformity in the crystalline condition ofthe polymer makes it dilficult to stretch the tube so that a film ofuniform gauge is obtained. In the continuous production of the film,therefore, we desire to form the tube, cool it to a crystalline stateand then reheat it to the orientation temperature.

This sequence of operations is illustrated in FIGURE 1.

A tube of the desired diameter and wall thickness is formed in extrusionstep 10 from the polymer melt. Extrusion temperatures will varyconsiderably depending upon the polymer used. For example, for polymerssuch as high density polyethylene or polypropylene extrusion which canthen be wound up on a reel in step 19.

Having thus described the overall operation in a general fashionattention is now given to the individual features, referring first toFIGURE 2. FIGURE 2 is an ele vational view of the extrusion, sizing andcooling stages of the operation. Molten polymer is fed in theconventional manner by extruder 20 to crosshead die 21. Crosshead die 21is equipped with a mandrel 22 and die 23 which together define anannular orifice through which the molten polymer is extruded in the formof a tube. The diameter and thickness of the tube thus extruded dependsupon the desired size and thickness of the expanded and oriented tubeand the degree of drawdown and expansion required to produce the desiredphysical properties.

As the tube issues from the die it passes immediately into cooling andsizing sleeve 24 which with jacket 26 is attached through collar 27 todie head 21. Jacket 26 defines annular chambers 28 and 29 through whichcooling liquid can be circulated in indirect heat exchange relationshipwith the. tube passing through sleeve 24. In order to facilitate theoperation on start-up and to insure that the tube makes close contactwith the walls of sleeve 24, a plurality of vacuum ports 30 are providedwith numerous holes connecting the ports to the space between tube Walland the cooling sleeve. Since there is usually a,

slight tendency of the tube to shrink as it is cooled, flange 31 withseal ring 32 is provided to seal the space between the tube wall and thecooling sleeve thereby preventing loss of the vacuum. A plurality ofO-rings 33 are provided between the jacket and the cooling sleeve inorder to seal the annular spaces used for vacuum and cooling liquid.

By the time tube 34 leaves the cooling sleeve it has been sufficientlycooled on the surface that it can be further cooled by a direct heatexchange with a cooling liquid. Tube 34 then passes directly into waterbath 36 through which water is circulated via inlet 37 and outlet 38.Flexible seals 39 and 40 at the entrance and exit, respectively, ofwaterbath 36 prevent the water from being lost from the tank. Thus, the tube34 is formed having the desired dimensions and with the polymer thereinin uniform crystalline condition. In the manufacture of film for heavybag material the tube will ordinarilyhave a diameter of about 2 to 6inches and a thickness in the range of about 30 to 70 mils.

Once the operation has been started and is on a continuous basis thegauge uniformity of the tube can be improved by employing relativelyhigh internal pressures Within the tube while it is in the cooling andsizing sleeve. Since relatively low pressures are necessary for theexpansion of the tube during the orientation process, we have providedthe apparatus shown in FIGURE 2 so that two distinct pressure zones canbe maintained within the tube; an upstream high pressure zone forexpanding the tube slightly against the walls of the sizing sleeve, anda downstream low pressure zone used for the orientation process. Thesetwo zones are maintained by seal 41 which is positioned within the tubedownstream from the sizing sleeve but upstream from the reheatingoperation. Conduit 42 passes axially through crosshead die '21 and isconnected to line 43. Line 43 contains pressure gauge 44 and pressureregulator 46 and is connected to a source of high pressure air throughconduit 47 and filter 48. Ordinarily a pressure in the range of about to30 lbs. per square inch gauge will be satisfactory for the purpose ofexpanding the tube against the walls of the sizing sleeve. I

Conduit 42 is in open communication with the upstream zone 49 within thetube between mandrel 22 and seal 41. Seal 41 prevents the high pressureWithin zone 49 from being transmitted to the volume within the tubedownstream from seal 41. Conduit 50, passes through seal 41 and axiallythrough conduit 42. Conduit 50 communicates with the zone within thetube downstream from seal 41 and is connected through line 51 containingpressure gauge 52 tothree-way valve 53. During normal operation line 51is connected through valve 53 to line 54 carrying pressure regulators56and 57 and pressure gauge 58. Conduit 54 is also connected through line47 to the high pressure air source but the pressure within line 54 atthe three-way valve 53 is reduced to about 1 to 3 lbs. per square inchgauge by regulators 56 and '57. Thus the pressure within zone 49 can bemaintained at about 10 to 30 lbs. per square inch for the purpose ofsizing the tube in cooling sleeve 24 while the pressure for theorientation operation is maintained much lower, for example about 1 to 3lbs. per square inch gauge. Where higher pressures are needed forinitially expanding the tube in starting up the orientation process,three-way valve 53 is provided so that line 51 can be manually connectedto the high pressure air source through line 59.

Referring now to FIGURE 3, a contoured jaw tube puller 60 is shown forthepurpose of pulling the tube from the cooling sleeve and through thewater bath and pushing the tube into the reheating bath 61. The speed oftube puller 60 is regulated so that the tube is pulled from the sizingsleeve slightlyfaster'than the rate. at which the tube is extruded fromthe die. The slight tension which is placed on the tube within thesizing sleeve causes a small reduction in tube thickness immediatelyafter the tube is extruded and before it is cooled and thereby improvesthe gauge uniformity of the tube. Tube puller 60 is provided with aplurality of contoured jaws 62 mounted in upper and lower chain sets 63and 64, respectively. Chain set 63 is driven by sprocket wheels 66 whichin turn are powered by a variable speed motor not shown. Chain set 63also turns on idler sprocket wheels 67. Chain set 64 is driven bysprocket'wheels 68 which are geared to sprocket wheels 66. Chain set 64also turns over idler sprocket wheels 69. As chain sets 63 and 64 arerotated the contour jaws 62 close about the tube 34 gripping it firmlybut without deformation and advance it from the water bath into thereheating bath 61. An-end viewof the contour jaw tube puller is shown inFIGURE 4. In order to prevent slipping and deformation of the tuberesilient pads 70 are provided in each of the contour jaws.

Referring again to FIGURE 3 tube 34 which is in a relatively cooluniform crystalline condition is passed by the tube puller 60 intoheating bath 61. Heating bath 61 comprises an elongated cylindricalshell 71. Shell 71 is fastened at one end by flange 72 having aresilient ring portion 73 to head member74. Head member 74 is equippedwith a heating liquid inlet 76 and a liquid seal 77 made of rubber orTeflon. This seal prevents the heating liquid, which is preferablyethylene glycol, from leaking at the point at which .the tube 34 entersthe heating bath. In a similar manner the shell 71 is attached at itsother end by flange 78 to head member 79 which is equipped with theglycol outlet 80 and liquid seal 81. A seal ring ofpolytetrafluoroethylene is preferred for this service. The insidediameter of shell 71 is larger than the outside diameter of tube 34 andthe tube is supported within the shell by helical rod 82. Rod 82 can beformed from metal and coated with polytetrafluoroethylene in order toreduce-friction between the rod and the outside of the tube. The outsidediameter of the helix corresponds approximatelyto the internal diameterof shell 71 and the internal diameter of the helix is approximatelyequal to the external diameter of tube 34. There is defined, therefore,by helical rod 82 and shell 71 cooperating with tube 34a helical pathpassing from the inlet end of shell 71 to the outlet thereof. encirclingthe tube 34 thereby insuring more uniform heating of the tube. Thetendency of the heating fluid to stratify according to temperature islessened and the temperature of the tube issuing from the heating bathis much more uniform.

1 Another very important advantage accrues from the use of the helicalshaped guide rod. This rod tends to act like a spring and compressslightlyon occasions when the tube starts to buckle within the bath.This slight compression of the guide rod prevents what would otherwisebecome a serious block-up in the heating bath requiring completeshut-down of the operation. The situation frequently corrects itself orcorrective action can be taken duringthe delay provided by thecompression of the guide rod. During normal operation, the slight springaction of the helix serves to maintain the friction drag on the tube ata low constant value by a self regulating action. If friction-were toincrease slightly, the increased drag of the tube on the helix wouldgive it a minute compression which minutely increases the insidediameter of the helix which in turn at once lowers the drag and thehelix loses its compression and the normal condition is restored.

We have found that improved uniformity of heating of the tube can beaffected through the use of a helical wiper 83 attached to rod 82. Onlya portion of wiper 83 is shown in FIGURE 3. This wiper is formed fromresilient material such as rubber which is resistant to the hot heatingfluid. Wiper 83 improves the seal between rod 82 and tube 34 therebyforcing better circulation of the heating fluid in the above describedhelical path. Also wiper 83 repeatedly wipes the liquid film from theouter surface of tube 34, thereby bringing about more eflicient heatexchange between the tube and the heating fluid.

Because of the tendency of the tube to buckle, as described above, it ishighly desirable to construct shell 71 from a transparent material suchas Pyrex glass. By so doing the condition of the tube withinthe heatingbath is clearly visible to an operator. There is frequently a tendencyfor the tube to block up within the heating bath, thereby necessitatingthe shut-down of the operation and repeating the involved start-upprocedure. We discovered that these block-ups were caused by the tubebuckling within the bath as a result of an imbalance between the rate atwhich the tube is forced into the bath and the rate at whichit iswithdrawn. 'By constructing the shell of a transparent material thistendency to buckle can be detected visually at a very early stage andthe take-off rate can be increased slightly to avoid the problem.

The residence time of the tube within the heating bath .must .besufiiciently long that all of the polymer in the tube is brought toorientation temperature. This does not mean that the temperature of thetube need be uniform throughout but there should not be more than a fewdegrees, for example, 1 to 5 F., difference between the inside and theoutside of the tube. Depending upon the operation, the length of heatingbath 61 can be increased or, as is frequently desirable, a plurality ofsuch heating baths can be used so that the temperature gradient of theheating liquid between its inlet and outlet is minimized. From apractical standpoint, the length of the heating bath is limited by thefriction between the tube and the guide rod. Necking of the tube withinthe bath must be avoided since otherwise the seal between seal ring 81and the tube cannot be maintained and the heating fluid will leak fromthe bath.

The heating liquid can pass in either concurrent or countercurrent flowto the travel of the tube but concurrent flow is preferred. If thetemperature in the bath is too high there is a tendency of the tube tostick to the helical rod or to the seal 81. If the temperature of thebath is too low there is too little heat transfer between the bath andthe tube. Ordinarily, the operation can be carried out so that theexternal surface temperature of the tube as it issues from the bath issubstantially the same as the temperature of the heating liquid in thebath and the internal temperature of the tube is within about 1 to 5 F.below the outside surface temperature. The heating liquid is wiped fromthe surface of the tube by seal 81 and the tube is then in the propercondition for biaxial orientation.

Referring now to FIGURE 5, tube 34 as it issues from the heating bath 61is expanded by internal fluid pressure while at the same time it isstretched in a linear direction. The trapped bubble method of operationis not adequate here but the inflating gas must be in continuous supplyand adequately pressured as described in connection with FIGURE 2. Theratio of the final to the initial diameter of the tube depends upon theproperties desired in the finished product. When working with tubes ofhighly crystalline olefin polymers, a clear, strong film can be producedusing relatively high blow-up ratios, for example, from about 7 to 1 to-10 to 1. We have found, however, that the tough films which are mostsuitable for the production of-bag material are made using much lowerblow-up ratios, for example about 3 to 1 to 6 to 1 and preferably ablow-up ratio of about 4 times is employed. For balanced properties theamount of stretch in both the machine and transverse directions shouldbe approximately equal. Some improvement in gauge uniformly can beobtained, however, if the machine direction stretch ratio is slightlyhigher than the transverse direction blow-up ratio.

The temperature at which the orientation is carried out is dependentupon the polymer employed. Using an ethylene polymer having a density ofabout 0.960 gram per cubic centimeter at 25 C., the orientation shouldbe carried out at a temperature in the range of 260 to 270 --F.,preferably in about the middle of this range. Better gauge uniformitycan thereby be obtained than when operating at somewhat lowertemperatures. These temperatures refer to the temperature of the polymerimmediately after it issues from the heating bath when stretchingbegins. We have found that once stretching has started it will proceedsatisfactorily at progressively lower temperatures. The best balance ofproperties can be obtained, therefore, by directing a cooling gas on theoutside of the expanding tube so that the temperature of the tubedecreases while it is undergoing the biaxial orientation. As shown inFIGURE 5, this cooling air is supplied tangentially at inlet 84 to openring member 86 which is positioned immediately downstream from theheating bath 61 so that the tube must pass through ring 86 as itexpands. In the absence of cooling gas supplied by ring member 86 thereis a tendency of the temperature of the film undergoing biaxialorientation to rise because of the work being performed on it. Becausethe tube as it issues from the heating bath is immediately atorientation temperature there would apparently be no need to conditionthe tube further temperaturewise. The stretching takes place immediatelyafter the tube issues from the heating bath so that this portion of theoperation is carried out in a relatively short distance, for example,about 2 to 10 inches, depending upon the diameter to which the tube isinflated. Even though the ambient atmosphere it at a temperature farbelow that of the tube as it issues from the bath, we have found that nosignificant cooling of the tube occurs in the absence of a direct effortto circulate cooling gas about the tube. The stagnant air filmeffectively insulates the expanding tube and, in any event, the heatloss to the surrounding atmosphere does little more offset the heatgenerated within the tube as a result of the mechanical work performedon it. The production of a decrease in temperature along the tube as itexpands was found to be essential to obtain the satisfactory balance ofproperties which is desired in bag materials.

After the tube has expanded to the desired diameter it passes into afinal sleeve 87 where it is chilled by cooling liquid circulatingthrough coils 88. Sleeve 87 is preferably aluminum with a chrome platepolished to a satin finish. The expanded tube is cooled sufficiently insleeve 87 that further stretching is prevented in either direction. Inplace of cooling sleeve 87, jets of cooling gas may be used to chill thetube to temperatures far below that necessary for orientation andthereby prevent further radial or longitudinal stretching. This finalcooling step must not be confused with the cooling air impinged upon theexpanding bubble by air ring 86. The cooling gas distributed by ring 86produces a cooling gradient across the expanding tube but maintain thetube at orientation temperature. The cooling which is carried by sleeve87 or equivalent means cools the tube after orientation has beencompleted and serves to set the orientation and prevent furtherexpansion. Thus,.the cooling functions illustrated in FIGURE 5 areindependent and each serves a different purpose.

Expanded and oriented film which is to be used for bags will ordinarilyhave a thickness of about 1 to 5 mils and the diameter of the tube mayvary from about 8 to 24 inches. Of course, other combinations ofdimensions are possible and depend upon the use to which the film isput. The expanded and oriented film passes from chilling sleeve 87 tocollapsing stand 89 which comprises upper and lower roller bearings 90and 91, respectively, which converge towards pinch rolls 92. Pinch rolls92 seal the expanding air within the tube and are power driven in orderto place the necessary tension on the tube required for the longitudinalstretching and orientation. The speed of pinch rolls 92 is adjusted sothat the take off rate of the film is faster than the rate at which thetube issues from the heating bath 61. The ratio of these two speedsdetermines the machine direction stretch ratio. The collapsed tube thenpasses over idler rolls 93 and 94 and between a second set of pinchrolls 94 before it is taken up on reel 97.

10 orientation. As has been pointed out these lower temperatures can beachieved once the stretching is initiated without sacrifice of gaugeuniformity. In order to minimize streaking of the film the cooling airshould be kept Another embodiment of this invention relative to the awayfrom the smaller portion of the bubble as it issues control of thetemperature of the expanding bubble is il from the heating bath. Theannular baflies can be used lustrated in FIGURE 6. This drawingillustrates how for this purpose to direct and confine the cooling airto the configuration of the expanding bubble can be concertain areas ofthe expanding tube. Thus by proceeding trolled and the areas which arecontacted by the cooling according to the invention, film can beproduced having gas varied by using a plurality of annular baffles 98,99, moderately high tensile strengths plus high elongation, 100 and 101.These bafiies, mounted on rods not shown, particularly in the machinedirection. This produces a are supported from cooling sleeve 87 so thatthe bafiles can tough film which is useful as a bag material and canwithbe moved back out of contact with the expanding bubble stand impactwhich is more important than high absolute against heating bath 61, orpositioned at various locations values of tensile strength. The propertyof machine dito efiect the impingement of the cooling air on certainrection elongation is especially important in applications areas of thetube as it expands. Bafiie 99 is shown where the bags are to be heatsealed as this elongation equipped with a ring member 102, having atangential air enables stress concentrations at the seal to bedissipated. inlet 103. This baflie can then serve the purpose similar Inorder to illustrate further the advantages of our into that of ring 86in FIGURE 5 and by moving the baffle vention, the following example ispresented. The condito various positions in relation to the otherbaffies the tions given in this example should be interpreted as typicaleffect of the cooling gas can be localized on various parts only and notconstrued to limit our invention unduly. of the expanding bubble. Thepositioning of the baffies Polyethylene having a density of 0.960 gramper cubic can be determined by distances a, b, c and d. Thisdiscentimeter and a melt index of 0.2 (ASTM-D-1238-52T) tance over whichthe bubble undergoes expansion is desigwas extruded at a temperature of350 F. through a 1 /2 nated by the distance x and it is over thisdistance that inch diameter die opening with an extruder screw speed thetemperature gradient is produced according to our of 22 r.p.m. Thethroughput of the extruder was 11 lbs. invention. This can also bedefined as the distance beper hour. The tube thus formed was passedthrough a tween the position where the tube begins inflation from ngSleeve, Cooled in a water bath and then Passed into diameter y and theposition where it stops inflating at aglycol heating bath in which theglycol flow was counterdiarneter z. In the biaxial orientation of highdensity current to the polymer tube. The inlet temperature of ethylenepolymers the temperature gradient should be at the glycol was 266 F. andthe outlet temperature was least 6 F. The cooling gradient should notexceed about 260 F. Polymer was thus heated to approximately the 20 F.If the stock is cooled too severely the surface inlet temperature of theglycol and then stretched biaxially o es tOO cold for proper stretchingand a rough apby inflation and simultaneous tension to give a machinepearing film results. direction stretch ratio of 6.2 and a transversedirection Theuse of the cooling gas on the expanding bubble acstretchratio of 4.3. The transverse direction stretch cording to our inventionenables greater flexibility between ra s the ratio of the final t-llhediameter to the diameter the relative ratios of machine direction andtransverse di- 0f the eXtfIlded tllhe- The machine direction Stretchrection stretching. The machine direction stretching canratio is theratio between the film Wihdup Speed and the not be increased merely byspeeding up the take-up rate Speed Of the tube Pllher upstream from theglycol hathwithout compensation in other variables of the process Thefinal film thickness was approximately as such action would merely causethe tube to neck in the llhlaf hflfhes Were used as Shown in FIGURE 6and the glycol bath causing the glycol to leak around the tube asspacings Were Changed as indicated in Table I With it leaves the bath orcausing the tube to split rather efehee t0 the Spaces lettered in FIGUREThe runs th n expand if l h use f the annular b ffi were carried outunder otherwiseidentical conditions exill d i FIGURE 6 provides aretarding force at cept for the presence or absence of cooling air asindicated the point of blowing since the tube tends to billow someinTable ling air, when employed was distributed what between the baffles.This relieves the extra tenaround the expanding bubble as shown n FIGURE6 sion from the tube in the bath and thereby limits the using an inletair Pressure of P qhleh gaugetension on the tube immediately downstreamfrom the The temperature of the cooling air Wes about bath. As shown inFIGURE 6, the tube is forced to as- The cooling gradient was thusproduced across the eX- sume a substantially conical shape as itexpands. Higher panding bubble in those runs where cooling air wasemmachine direction stretching is thereby possible through ployed.

TABLE I Baflfieianiplacmg Cod ng Tensigglssfiength Elliliggcaggn,

Air

a b c (1 MD TD MD TD ,4 A %0 34 None 22,700 15,200 60 110 t4 at so 4 Yes10, 400 16,100 90 V1 916 $1 None 24,000 10,400 40 V2 V1 A0 Yes 15,20014,200 60 ts m None 23, 700 11,600 40 M 91s Yes 15,400 12, 000 70 100the use of the annular baflies. Also by regulating the The data of TableI show that the use of cooling air cooling bubble as it expands,balanced film properties permitted more balanced tensile properties inboth macan be obtained with less restriction on the operating 70 chineand transverse directions and also better balanced rates. The coolinggas tends to shift the machine direcelongation. The substantiallyincreased elongation in the n stretch to the hotter p r p r n f hebubble machine direction is desirable for the formation of bags wherethe stretching produces less orientation. This also t b h t l d causesthe transverse stretching to take place at lower As will be apparent tothose skilled in the art from the temperatures where the stretching ismore efiective for 75 above disclosure, various modifications can bemade in 1 1 our invention without departing from the spirit or scopethereof.

We claim:

1. Apparatus for producing a biaxially oriented film of crystallinethermoplastic polymer comprising, in combination, extrusion meansincluding a tubing die, a sizing and cooling sleeve attached to saiddie, a cooling bath positioned to receive the extruded tube from saidsleeve, means for pulling said tube through said sleeve and coolingbat-h, a heating bath positioned downstream from said pulling means,means for circulating a heated liquid through said heating bath in orderto bring the tube to orientation temperature, means for introducingpressurized gas inside the tube in order to inflate same as it issuesfrom said heating bath, means for collapsing and pulling the inflatedtube in order to stretch the tube lengthwise as it issues from saidheating bath, means for chilling the inflated and stretched tube inorder to set the orient-ation prior to collapsing the tube and meanscomprising a plurality of annular baffles of graduated inside diameterspositioned between said heating bath and said chilling means in spacedrelationship to define a substantially conical path for the tube as itis inflated (for directing a current of cooling gas onto the outersurface of the tube as it is inflated and stretched).

2. Apparatus of claim 1 wherein a tube sealing means is disposed betweensaid sleeve and said heating means thereby forming distinct upstream anddownstream zones within the tube passed around same and wherein a firstconduit extends through said die head and connects a source of highpressure gas to said upstream zone and second conduit extends throughsaid die head, upstream zone and tube sealing means and connects asource of low pressure gas to said downstream zone.

3. Apparatus for producing a biaxially oriented film of crystallinethermoplastic polymer comprising, in combination, extrusion meansincluding a tubing die, a sizing and cooling sleeve attached to saiddie, a cooling bath positioned to receive the extruded tube from saidsleeve, means for pulling said tube through said sleeve and coolingbath, a heating bath positioned downstream from said pulling means,means for circulating a heated liquid through said heating bath in orderto bring the tube to orientation temperature, means for introducingpressurized gas inside the tube in order to inflate same as it issuesfrom said heating bath, means for collapsing and pulling the inflatedtube in order to stretch the tube lengthwise as it issues from saidheating bath, means for chilling the inflated and stretched tube inorder to set the orientation prior to collapsing the tube andmeans'comprising an annular chamber through which the tube passesbetween said heating bath and said chilling means and means fordistributing a stream of cooling gas inside said annular chamber aroundsaid tube for directing a current of cooling gas onto the outer surfaceof said tube as it is inflated and stretched.

4. The apparatus of claim 3 wherein a tube sealing means is disposedbetween said sleeve and said heating means thereby forming distinctupstream and downstream zones within the tube passed around same andwherein a first conduit extends through said die head and connects asource of high pressure gas to said upstream zone and second conduitextends through said die head, upstream zone and tube sealing means andconnects a source of low pressure gas to said downstream zone.

5. Apparatus for heating a tube of thermoplastic polymer for biaxialorientation comprising an elongated cylindrical shell through which saidtube can be passed, a helical rod disposed longitudinally within saidshell, the outside diameter of the helix approximately equalling theinside diameter of said shell and the inside diameter of said helixequalling substantially the outside diameter of the tube so that saidrod and shell cooperate with the tube to define a helical channelpassing through said shell between the shell and the tube, means forintroducing a heating liquid into one end of said channel, means forremoving said liquid from the outer end of said channel, and sealingmeans at each end of said shell to prevent said liquid from escaping atthe entry and exit of said tube.

6. The apparatus of claim 5 wherein said shell is transparent permittingview of said tube within said shell.

7. The apparatus of claim 5 comprising a resilient wiper attached alongsaid rod to make helical contact with said tube.

8. The apparatus of claim 7 wherein said helical rod is flexible and hasfreedom of longitudinal movement in compressive action within said shelland said outside diameter of the helix is slightly smaller than saidinside diameter of said shell so that said helix is capable of a smallaxial compression due to the frictional drag of said tube with aresulting slight increase in said inside diameter of said helix.

References Cited by the Examiner UNITED STATES PATENTS 2,947,031 8/1960H0 Chow, et al 1814 X 2,947,032 8/1960 Taylor 1814 2,955,321 11/1960Former et al 18-14 X 2,963,742 12/1960 Ahlich et al. 18-14 3,068,51612/1962 Hofer 18-14 X 3,217,359 11/1965 Euling 18l4 FOREIGN PATENTS1,238,367 7/1960 France.

WILLIAM J. STEPHENSON, Primary Examiner.

1. APPARATUS FOR PRODUCING A BIAXIALLY ORIENTED FILM OF CRYSTALLINETHERMOPLASTIC POLYMER COMPRISING, IN COMBINATION, EXTRUSION MEANSINCLUDING A TUBING DIE, A SIZING AND COOLING SLEEVE ATTACHED TO SAIDDIE, A COOLING BATH POSITIONED TO RECEIVE THE EXTRUDED TUBE FROM SAIDSLEEVE, MEANS FOR OUTLING SAID TUBE THROUGH SAID SLEEVE AND COOLINGBATH, A HEATING BATH POSITIONED DOWNSTREAM FROM SAID PULLING MEANS,MEANS FOR CIRCULATING A HEATED LIQUID THROUGH SAID HEATING BATH IN ORDERTO BRING THE TUBE TO ORIENTATION TEMPERATURE, MEANS FOR INTRODUCINGPRESSURIZED GAS INSIDE THE TUBE IN ORDER TO INFALTE SAME AS IT ISSUESFROM SAID HEATING BATH, MEANS FOR COLLAPSING AND PULLING THE INFALTEDTUBE IN ORDER TO STRETCH THE TUBE LENGTHWISE AS IT ISSUES FROM SAIDHEATING BATH, MEANS FOR CHILLING THE INFLATED AND STRETCHED TUBE INORDER TO SET THE ORIENTATION PRIOR TO COLLAPSING THE TUBE AND MEANSCOMPRISING A PLURALITY OF ANNULAR BAFFLES OF GRADUATED INSIDE DIAMETERSPOSITIONED BETWEEN SAID HEATING BATH AND SAID CHILLING MEANS IN SPACEDRELATIONSHIP TO DEFINE A SUBSTANTIALLY CONICAL PATH FOR THE TUBE AS TIIS INFLATED (FOR DIRECTING A CURRENT OF COOLING GAS ONTO THE OUTERSURFACE OF THE TUBE AS IT IS INFLATED AND STRETCHED).